Over 1.22 million new cancer cases will be diagnosed in the U.S. in the year 2001 alone. With more than 563,000 deaths annually, cancer is the second leading cause of death behind heart disease (UBS Warburg “Disease Dynamics: The Cancer Market”, Nov. 8, 2000). Surgery and radiotherapy may be curative if the disease is found early, but current drug therapies for metastatic disease are mostly palliative and seldom offer a long-term cure. Even with the new chemotherapies entering the market, improvement in patient survival is measured in months rather than in years, and the need continues for new drugs effective both in combination with existing agents as first line therapy and as second and third line therapies in treatment of resistant tumors.
In the past, the most successful drug treatment regimens have combined two or more agents, each of which has a different mechanism of action and each of which has antitumor activity when used individually. Even though their mechanisms of action differ, most of the agents currently used for chemotherapy of cancer, including alkylating agents, platinum analogs, anthracyclines and the camptothecin family of topoisomerase inhibitors, have in common the property of severely damaging DNA, hence their designation as “DNA-damaging agents”. Radiotherapy works similarly. Most DNA-damaging agents as well as the microtubule-targeting agents (e.g., paclitaxel) cause the arrest of cells at the G2/M transition phase of the cell cycle, a major cell cycle checkpoint where cells make a commitment to repair DNA or to undergo apoptosis if DNA damage al., Molecular and Biochemical Parasitology 1:167–176 (1998) (substituents at the 2- and 3-positions)).
As a single agent, β-lapachone has demonstrated significant antineoplastic activity against human cancer cell lines at concentrations typically in the range of 1-10 μM (IC50). Cytotoxicity has been demonstrated in transformed cell lines derived from patients with promyelocytic leukemia (Planchon et al., Cancer Res., 55 (1996) 3706), prostate (Li, C. J., et al., Cancer Res., 55 (1995) 3712), malignant glioma (Weller, M. et al., Int. J. Cancer, 73 (1997) 707), hepatoma (Lai, C. C., et al., Histol Histopathol, 13 (1998) 8), colon (Huang, L., et al., Mol Med, 5, (1999) 711), breast (Wuertzberger, S. M., et al., Cancer Res., 58 (1998) 1876), ovarian (Li, C. J. et al., Proc. Natl. Acad. Sci. USA, 96(23) (1999) 13369-74), pancreatic (Li, Y., et al., Mol Med, 6 (2000) 1008; Li, Y. Z., Mol Med, 5 (1999) 232), and multiple myeloma cell lines, including drug-resistant lines (Li, Y., Mol Med, 6 (2000) 1008). No cytotoxic effects were observed on normal fresh or proliferating human PBMC (Li, Y., Mol Med, 6 (2000) 1008).
β-lapachone has been shown to be a DNA repair inhibitor that sensitizes cells to DNA-damaging agents including radiation (Boothman, D. A. et al., Cancer Res, 47 (1987) 5361; Boorstein, R. J., et al., Biochem. Biophys. Commun., 117 (1983) 30). Although its exact intracellular target(s) and mechanism of cell killing remain unknown, β-lapachone has also shown potent in vitro inhibition of human DNA Topoisomerases I (Li, C. J. et al., J. Biol. Chem., 268 (1993) 22463) and II (Frydman, B. et al., Cancer Res. 57 (1997) 620) with novel mechanisms of action. Unlike topoisomerase “poisons” (e.g., camptothecin, etoposide, doxorubicin) which stabilize the covalent topoisomerase-DNA complex and induce topoisomerase-mediated DNA cleavage, β-lapachone interacts directly with the enzyme to inhibit catalysis and block the formation of cleavable complex (Li, C. J. et al., J. Biol. Chem., 268 (1993) 22463) or with the complex itself, causing religation of DNA breaks and dissociation of the enzyme from DNA (Krishnan, P. et al., Biochem Pharm, 60 (2000) 1367). β-lapachone and its derivatives have also been synthesized and tested as anti-viral and anti-parasitic agents (Goncalves, A. M., et al., Mol. Biochem. Parasitology, 1 (1980) 167–176; Schaffner-Sabba, K., et al., J. Med Chem., 27 (1984) 990–994).
More specifically, β-lapachone appears to work by disrupting DNA replication, causing cell-cycle delays in G1 and/or S phase, inducing either apoptotic or necrotic cell death in a wide variety of human carcinoma cell lines without DNA damage and independent of p53 status (Li, Y. Z. et al (1999); Huang, L. et al.). Topoisomerase I is an enzyme that unwinds the DNA that makes up the chromosomes. The chromosomes must be unwound in order for the cell to use the genetic information to synthesize proteins; β-lapachone keeps the chromosomes wound tight, so that the cell cannot make proteins. As a result, the cell stops growing. Because cancer cells are constantly replicating and circumvent many mechanisms that restrict replication in normal cells, they are more vulnerable to topoisomerase inhibition than are normal cells.
Another possible intracellular target for β-lapachone in tumor cells is the enzyme NAP(P)H:quinone oxidoreductase (NQO1). Biochemical studies suggest that reduction of β-lapachone by NQO1 leads to a “futile cycling” between the quinone and hydroquinone forms with a concomitant loss of reduced NADH or NAD(P)H (Pink, J. J. et al., J. Biol Chem., 275 (2000) 5416). The exhaustion of these reduced enzyme cofactors may be a critical factor for the activation of the apoptotic pathway after β-lapachone treatment.
As a result of these findings, β-lapachone is actively being developed for the treatment of cancer and tumors. In WO00/61142, for example, there is disclosed a method and composition for the treatment of cancer, which comprises the administration of an effective amount of a first compound, a G1 or S phase drug, such as a β-lapachone, in combination with a G2/M drug, such as a taxane derivative. Additionally, U.S. Pat. No. 6,245,807 discloses the use of β-lapachone, amongst other β-lapachone derivatives, for use in the treatment of human prostate disease.
One obstacle, however, to the development of pharmaceutical formulations comprising β-lapachone for parenteral and topical administration is the low solubility of β-lapachone in pharmaceutically acceptable solvents. β-lapachone is highly insoluble in water and has only limited solubility in common solvent systems used for topical and parenteral administration, specifically for intravenous and cutaneous delivery of drugs. As a result, there is a need for improved formulations of β-lapachone for parenteral and topical administration, which are both safe and readily bioavailable to the subject to which the formulation is administered.